The present invention relates to satellite communications systems. More specifically, the invention relates to a method of synchronizing received uplink signals at a satellite such that bulk signal processing may be used to simultaneously recover multiple data channels in the uplink signals.
Modern communications networks carry staggering amounts of information, typically divided for transmission purposes into individual data channels. Whether the data channels carried by the communications network have their origin in the telephone system, television stations, or other source, these data channels often need to be transmitted through a communications network including a satellite link.
Where a satellite is a link in the communications network, Customer Premises Equipment (CPE) is used to format the data channels for transmission to the satellite in an uplink signal. Multiple uplink signals may be generated by multiple CPEs, with each uplink signal carrying one or more data channels. Because many CPEs may communicate with a single satellite, the potential number of data channels that a single satellite may need to process can grow very large.
With standard frequency division multiple access (FDMA), the available bandwidth is divided into many narrow frequency subbands (channels) with each CPE transmitting on a unique frequency channel. In addition, the frequency responses of the channels must be kept separated to reduce adjacent channel interference (ACI) which can significantly degrade performance. In conventional satellites, each data channel present in an FDMA uplink signal is received and demodulated on an individual basis.
A very efficient bulk demodulation scheme can be implemented at the satellite to receive and demodulate all the channels in the uplink signal in a single operation. Bulk demodulation requires time signaling synchronization among all the CPEs transmitting uplink signals. In the past, however, satellites and CPEs have been unable to achieve synchronization among the individual CPEs.
Along with time and symbol synchronization, the channels can be spaced such that their frequency responses are orthogonal (thereby giving rise to the name orthogonal frequency division multiplexing (OFDM)). With orthogonal signaling, the frequency responses can be overlapping, and with proper processing, be free of ACI. The result is very efficient bandwidth utilization.
Today, tens of thousands of data channels to be processed by the communications satellite may compete for services in an uplink OFDM signal. A single satellite would require enormous amounts of space, weight, and power to receive, demodulate, and decode the unsynchronized data channels on an individual basis. Increasing the size, weight, and onboard power of a satellite, of course, drives up the cost of the satellite dramatically.
The satellite itself becomes more expensive because of the large quantity of additional processing circuitry required to process data channels on an individual basis. Furthermore, larger, heavier, and more costly solar panels, batteries, or other power sources are required to provide onboard power. In addition, the satellite costs much more to launch because larger rockets using greater quantities of propellant are required to put the larger and heavier satellite into orbit.
Thus, in the past, satellite size, weight, and power restrictions have prohibited satellites from processing the large numbers of data channels that modern communications techniques can generate. A need is present in the industry for improved satellite communications techniques, which overcomes the disadvantages discussed above and previously experienced.